1. Field of the Invention
The present invention relates to a method of manufacturing optical articles of high precision and high quality by press molding, which considerably lengthens the lifetime of molds being used.
2. Explanation of the Prior Art
Many methods of manufacturing optical articles have been proposed. An example among them is disclosed in Japanese patent laid-open publication HEI 3-50126: a preform having a given shape is placed between a pair of molds which have a molding surface precisely shaped in the reverse of the desired optical article to form, the preform being heated to a temperature at which it can be reshaped, then being pressed to form a desired optical article, and finally being cooled down to the room temperature. An example of a preform for use in the molding is described in the Japanese patent laid-open publication SHO 60-246231 as a glass cylinder cut to a given length, both the end surfaces being polished to a mirror finished surface as shown in FIG. 11. The Japanese patent laid-open publication SHO 61-261225 describes another example of a preform, which is a block of glass shaped to a sphere as shown in FIG. 12.
In the meanwhile, the lifetime of molds is a very important factor in the press molding of optical articles. Namely, the cost for manufacturing molds becomes very high since they need a very high precision finishing to mold optical articles of a high precision. The price of each optical article press molded includes a fraction of the manufacturing cost of molds naturally. Thus, the cost per optical article to be charged becomes lower as the lifetime of molds becomes longer.
Conventionally, special cares have been paid for the hardness of a preform material and protection layer formed on a molding surface to enhance the lifetime of molds. Since the molding surface contacts with a preform heated up to a high temperature, it must be covered by a protection layer which is hardly oxidized and inactive to the preform material such as glass.
Material for molds must have a hardness enough for maintaining a high working precision against forces exerted from the preform. As to the material for molds, cemented carbides or metals including tungsten are used satisfactorily and platinum film is used for the protection layer. The high precision press molding of optical articles becomes available by development of material for molds and protection layer mentioned above.
The mold having a protection layer is gradually deteriorated in its shape precision by use for a long time and the surface of the protection layer is roughened by continual contact with glass of a high temperature. Optical articles thus press molded are deteriorated in the shape precision gradually and, finally, become impossible to satisfy the specification thereof. In actual use, the lifetime of molds is estimated so at a time a little bit before optical articles become unsatisfactory and is indicated by the number of good optical articles producible thereby.
Conventionally, it has been considered that one of the main factors dominating the lifetime of molds is deterioration in shape of molds. Accordingly, trials for lengthening the lifetime of molds and, thereby, reducing the price of optical articles are directed to development of new materials for molds.
Inventors of the present invention found out the fact that the lifetime of molds was dominated by relationship between a shape of a molding surface and that of a preform. This will be explained in detail below.
Consider a press molding of cylindrical glass preforms as shown in FIG. 11.
At the first stage of the press molding, top and bottom molds 11 and 12 having molding surfaces 11a and 12a, respectively, contact with circular edges A and B of a cylindrical preform 14 at first. At this stage, a press force against the preform is maintained at a small value to avoid the preform from breaking or cracking. As the molding progresses with increase of contact areas between the molding surface and the preform, the press force is gradually increased and, at the final stage, a full press force is applied to deform the preform as a whole. At the first stage, edges A and B abut to the same portions of molds always. Thus, concentrated stresses are repeatedly applied to the same portions of molds resulting in earlier deterioration of the same portions. In fact, minute concave defects are caused at the specific portion of the molding surface and they are transferred as minute convex defects to a surface of an optical article upon press molding. Thus, the lifetime of molds is determined by generation of these defects substantially.
In the case as shown in FIG. 12, point C on the molding surface receives a concentrated stress repeatedly and, accordingly, a defect is caused thereat.